I Got The Nexplanon In March 2015. I Was Breastfeeding

I got the nexplanon in march 2015. I was breastfeeding and didn't get a period until June and ever since then I get about 3 periods a month. Beginning middle and end usually about 3-7 days long. But this month I have not had a period at all and I got a clearly positive pregnancy test (first response) but I got a negative blood test. Still no period. What's going on?